Recently, the performance of computers in general is increasing and the size and weight is decreasing. Accordingly, the use of portable computers or mobile computers (hereafter referred to as information terminal unit) is spreading. Moreover, many types of communication means likes LAN (Local Area Network), ordinary public lines, radio lines {portable telephone line or PHS (Personal Handy phone System) line} are available for accessing a server using an information terminal unit. Because of the availability of such an environment, it has become very easy for a user to readily execute data communication with a server using his information terminal unit and a communication line.
However, the conventional mobile communication environment has problems as follows. The operability is very bad because many times the line suddenly gets disconnected due to deterioration of received radio-field intensity. Further, depending upon the type of the line, the operation is different. Because of such problems in the conventional mobile communication environment, means or a method for effectively solving the above problem is earnestly desired.
Information terminal units that can be connected to the office's server from outside using a line are finding great use in the business. The information terminal unit of this type is provided with a PCMCIA (Personal Computer Memory Card International Association) card slot into which a PCMCIA card such as a portable-telephone card or PHS card or the like is inserted.
A portable-telephone card serves as an interface between an information terminal unit and a portable-telephone terminal, which is inserted into a PCMCIA card slot when performing data communication between an information terminal unit and a server through a portable-telephone line by using a portable-telephone terminal. However, a PHS card serves as an interface between an information terminal unit and a PHS terminal, which is inserted into a PCMCIA card slot when performing data communication between an information terminal unit and a server through a PHS line.
In the case of the above configuration, a user confirms that a received radio-field intensity is kept at a communicable level through a level meter indicated on a display of a portable-telephone terminal (or PHS terminal) and then, inserts a portable-telephone card (or PHS card) into a PCMCIA card slot of an information terminal unit. Thereby, the information terminal unit is connected to the portable-telephone terminal (or PHS terminal) through the portable-telephone card (or PHS card).
Under the above state, when a start key is operated by an operator, the information terminal unit is connected to a server via a portable-telephone line (or PHS line) and data communication is performed with the server. Moreover, a connector section used for a LAN cable is provided for the information terminal unit in addition to the above PCMCIA card slot. Therefore, by connecting a LAN cable to the information terminal unit through the connector section, it is possible to perform data communication with the server through a LAN.
As described above, a conventional information terminal unit uses a radio line (portable-telephone line or PHS line). Therefore, a line is disconnected because a received radio-field intensity is deteriorated nearby a radio-wave shielding body such as a building or the like and transferred data is canceled. In this case, a user must move to a place at a high received radio-field intensity checking a level meter of a display and then, retry data communication. Therefore, the operability is bad. In this case, because data processing is executed from the beginning after retrying the data communication, there has been a problem that the processing time required for the data communication is increased when including the processing time before line disconnection. The above problem also occurs when data communication is interrupted through user's operation and then resumed.
Moreover, when considering a utilization mode of a conventional information terminal unit, a state also occurs that data communication is interrupted temporally by a user because a received radio-field intensity may be deteriorated while the data is performed through a PHS line and thereafter, the data is resumed through another line (e.g., a portable-telephone line). Also in this case, it is necessary to retry data communication through a portable-telephone line. Therefore, operability is bad. Moreover, the data transferred through a PHS line is canceled and protocols are changed from a communication protocol for a PHS line to a communication protocol for a portable-telephone line. Thereby, the processing time required for data communication is increased.
Moreover, in the case of a conventional information terminal unit, a user must confirm a received radio-field intensity and perform the operation for data communication whenever a portable-telephone card or a PHS card is inserted. Therefore, there is a problem that it is impossible to correspond to the needs for automation.